buildman: Add support for building a buildman PyPi package

Create the necessary files to build this new package.

It is not actually clear whether this is useful, since buildman has no
purpose outside U-Boot. It is included for completeness, since adding
this later would be more trouble.

Move the main program into a function so that it can easily be called by
the PyPi-created script.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 21e58ca..f4ffa9e 100644
--- a/Makefile
+++ b/Makefile
@@ -2283,6 +2283,7 @@
 _pip:
 	scripts/make_pip.sh u_boot_pylib ${PIP_ARGS}
 	scripts/make_pip.sh patman ${PIP_ARGS}
+	scripts/make_pip.sh buildman ${PIP_ARGS}
 
 help:
 	@echo  'Cleaning targets:'